A more detailed explanation is the surface normals of the cage dictates the angle in which the high poly is captured. If the lowpoly and cage are completely flat, the highpoly will be captured as completely flat. If there is a flat part on the low poly that leads into a sloped area, you'll see on the smoothed normals that…
im confused are you wanting to take photo details and make a normal map out of them, or do you just want to bake detials to a flat texture. since if you just want to bake highpoly details to a flat texture just bake to a flat plane.